All Atoms Are

Overview of Atomic Structure

Atoms are made up of particles called protons, neutrons, and electrons, which are responsible for the mass and charge of atoms.

Because all macroscopic objects are in fact combinations of atoms, everything in the macroscopic world is subject to change, as their constituent atoms shift or move away. Thus, while the atoms themselves persist through all time, everything in the world of our experience is transitory and subject to dissolution. All atoms have a dense central core called the atomic nucleus. Forming the nucleus are two kinds of particles: protons, which have a positive electrical charge, and neutrons, which have no charge. All atoms have at least one proton in their core, and the number of protons determines which kind of element an atom is. For example, an oxygen atom.

Same

Learning Objectives

Discuss the electronic and structural properties of an atom

Key Takeaways

Key Points

  • An atom is composed of two regions: the nucleus, which is in the center of the atom and contains protons and neutrons, and the outer region of the atom, which holds its electrons in orbit around the nucleus.
  • Protons and neutrons have approximately the same mass, about 1.67 × 10-24 grams, which scientists define as one atomic mass unit (amu) or one Dalton.
  • Each electron has a negative charge (-1) equal to the positive charge of a proton (+1).
  • Neutrons are uncharged particles found within the nucleus.

Key Terms

  • atom: The smallest possible amount of matter which still retains its identity as a chemical element, consisting of a nucleus surrounded by electrons.
  • proton: Positively charged subatomic particle forming part of the nucleus of an atom and determining the atomic number of an element. It weighs 1 amu.
  • neutron: A subatomic particle forming part of the nucleus of an atom. It has no charge. It is equal in mass to a proton or it weighs 1 amu.

An atom is the smallest unit of matter that retains all of the chemical properties of an element. Atoms combine to form molecules, which then interact to form solids, gases, or liquids. For example, water is composed of hydrogen and oxygen atoms that have combined to form water molecules. Many biological processes are devoted to breaking down molecules into their component atoms so they can be reassembled into a more useful molecule.

Atomic Particles

Atoms consist of three basic particles: protons, electrons, and neutrons. The nucleus (center) of the atom contains the protons (positively charged) and the neutrons (no charge). The outermost regions of the atom are called electron shells and contain the electrons (negatively charged). Atoms have different properties based on the arrangement and number of their basic particles.

The hydrogen atom (H) contains only one proton, one electron, and no neutrons. This can be determined using the atomic number and the mass number of the element (see the concept on atomic numbers and mass numbers).

Structure of an atom: Elements, such as helium, depicted here, are made up of atoms. Atoms are made up of protons and neutrons located within the nucleus, with electrons in orbitals surrounding the nucleus.

All Atoms Are Composed Of Several Types Of

Atomic Mass

Protons and neutrons have approximately the same mass, about 1.67 × 10-24 grams. Scientists define this amount of mass as one atomic mass unit (amu) or one Dalton. Although similar in mass, protons are positively charged, while neutrons have no charge. Therefore, the number of neutrons in an atom contributes significantly to its mass, but not to its charge.

Electrons are much smaller in mass than protons, weighing only 9.11 × 10-28 grams, or about 1/1800 of an atomic mass unit. Therefore, they do not contribute much to an element’s overall atomic mass. When considering atomic mass, it is customary to ignore the mass of any electrons and calculate the atom’s mass based on the number of protons and neutrons alone.

Electrons contribute greatly to the atom’s charge, as each electron has a negative charge equal to the positive charge of a proton. Scientists define these charges as “+1” and “-1. ” In an uncharged, neutral atom, the number of electrons orbiting the nucleus is equal to the number of protons inside the nucleus. In these atoms, the positive and negative charges cancel each other out, leading to an atom with no net charge.

Protons, neutrons, and electrons: Both protons and neutrons have a mass of 1 amu and are found in the nucleus. However, protons have a charge of +1, and neutrons are uncharged. Electrons have a mass of approximately 0 amu, orbit the nucleus, and have a charge of -1.

Exploring Electron Properties: Compare the behavior of electrons to that of other charged particles to discover properties of electrons such as charge and mass.

Volume of Atoms

Accounting for the sizes of protons, neutrons, and electrons, most of the volume of an atom—greater than 99 percent—is, in fact, empty space. Despite all this empty space, solid objects do not just pass through one another. The electrons that surround all atoms are negatively charged and cause atoms to repel one another, preventing atoms from occupying the same space. These intermolecular forces prevent you from falling through an object like your chair.

Interactive: Build an Atom: Build an atom out of protons, neutrons, and electrons, and see how the element, charge, and mass change. Then play a game to test your ideas!

Atomic Number and Mass Number

The atomic number is the number of protons in an element, while the mass number is the number of protons plus the number of neutrons.

Learning Objectives

Determine the relationship between the mass number of an atom, its atomic number, its atomic mass, and its number of subatomic particles

Key Takeaways

Key Points

  • Neutral atoms of each element contain an equal number of protons and electrons.
  • The number of protons determines an element’s atomic number and is used to distinguish one element from another.
  • The number of neutrons is variable, resulting in isotopes, which are different forms of the same atom that vary only in the number of neutrons they possess.
  • Together, the number of protons and the number of neutrons determine an element’s mass number.
  • Since an element’s isotopes have slightly different mass numbers, the atomic mass is calculated by obtaining the mean of the mass numbers for its isotopes.

Key Terms

All Atoms Are
  • mass number: The sum of the number of protons and the number of neutrons in an atom.
  • atomic number: The number of protons in an atom.
  • atomic mass: The average mass of an atom, taking into account all its naturally occurring isotopes.

All Atoms Are Neutral

Atomic Number

Neutral atoms of an element contain an equal number of protons and electrons. The number of protons determines an element’s atomic number (Z) and distinguishes one element from another. For example, carbon’s atomic number (Z) is 6 because it has 6 protons. The number of neutrons can vary to produce isotopes, which are atoms of the same element that have different numbers of neutrons. The number of electrons can also be different in atoms of the same element, thus producing ions (charged atoms). For instance, iron, Fe, can exist in its neutral state, or in the +2 and +3 ionic states.

Mass Number

An element’s mass number (A) is the sum of the number of protons and the number of neutrons. The small contribution of mass from electrons is disregarded in calculating the mass number. This approximation of mass can be used to easily calculate how many neutrons an element has by simply subtracting the number of protons from the mass number. Protons and neutrons both weigh about one atomic mass unit or amu. Isotopes of the same element will have the same atomic number but different mass numbers.

Friday night funkin on phone. Atomic number, chemical symbol, and mass number: Carbon has an atomic number of six, and two stable isotopes with mass numbers of twelve and thirteen, respectively. Its average atomic mass is 12.11.

Scientists determine the atomic mass by calculating the mean of the mass numbers for its naturally-occurring isotopes. Often, the resulting number contains a decimal. For example, the atomic mass of chlorine (Cl) is 35.45 amu because chlorine is composed of several isotopes, some (the majority) with an atomic mass of 35 amu (17 protons and 18 neutrons) and some with an atomic mass of 37 amu (17 protons and 20 neutrons).

Given an atomic number (Z) and mass number (A), you can find the number of protons, neutrons, and electrons in a neutral atom. For example, a lithium atom (Z=3, A=7 amu) contains three protons (found from Z), three electrons (as the number of protons is equal to the number of electrons in an atom), and four neutrons (7 – 3 = 4).

Isotopes

Isotopes are various forms of an element that have the same number of protons, but a different number of neutrons.

Learning Objectives

Discuss the properties of isotopes and their use in radiometric dating

Key Takeaways

Key Points

  • Isotopes are atoms of the same element that contain an identical number of protons, but a different number of neutrons.
  • Despite having different numbers of neutrons, isotopes of the same element have very similar physical properties.
  • Some isotopes are unstable and will undergo radioactive decay to become other elements.
  • The predictable half-life of different decaying isotopes allows scientists to date material based on its isotopic composition, such as with Carbon-14 dating.

Key Terms

  • isotope: Any of two or more forms of an element where the atoms have the same number of protons, but a different number of neutrons within their nuclei.
  • half-life: The time it takes for half of the original concentration of an isotope to decay back to its more stable form.
  • radioactive isotopes: an atom with an unstable nucleus, characterized by excess energy available that undergoes radioactive decay and creates most commonly gamma rays, alpha or beta particles.
  • radiocarbon dating: Determining the age of an object by comparing the ratio of the 14C concentration found in it to the amount of 14C in the atmosphere.

What is an Isotope?

Isotopes are various forms of an element that have the same number of protons but a different number of neutrons. Some elements, such as carbon, potassium, and uranium, have multiple naturally-occurring isotopes. Isotopes are defined first by their element and then by the sum of the protons and neutrons present.

  • Carbon-12 (or 12C) contains six protons, six neutrons, and six electrons; therefore, it has a mass number of 12 amu (six protons and six neutrons).
  • Carbon-14 (or 14C) contains six protons, eight neutrons, and six electrons; its atomic mass is 14 amu (six protons and eight neutrons).

While the mass of individual isotopes is different, their physical and chemical properties remain mostly unchanged.

Isotopes do differ in their stability. Carbon-12 (12C) is the most abundant of the carbon isotopes, accounting for 98.89% of carbon on Earth. Carbon-14 (14C) is unstable and only occurs in trace amounts. Unstable isotopes most commonly emit alpha particles (He2+) and electrons. Neutrons, protons, and positrons can also be emitted and electrons can be captured to attain a more stable atomic configuration (lower level of potential energy ) through a process called radioactive decay. The new atoms created may be in a high energy state and emit gamma rays which lowers the energy but alone does not change the atom into another isotope. These atoms are called radioactive isotopes or radioisotopes.

Radiocarbon Dating

Carbon is normally present in the atmosphere in the form of gaseous compounds like carbon dioxide and methane. Carbon-14 (14C) is a naturally-occurring radioisotope that is created from atmospheric 14N (nitrogen) by the addition of a neutron and the loss of a proton, which is caused by cosmic rays. This is a continuous process so more 14C is always being created in the atmosphere. Once produced, the 14C often combines with the oxygen in the atmosphere to form carbon dioxide. Carbon dioxide produced in this way diffuses in the atmosphere, is dissolved in the ocean, and is incorporated by plants via photosynthesis. Animals eat the plants and, ultimately, the radiocarbon is distributed throughout the biosphere.

In living organisms, the relative amount of 14C in their body is approximately equal to the concentration of 14C in the atmosphere. When an organism dies, it is no longer ingesting 14C, so the ratio between 14C and 12C will decline as 14C gradually decays back to 14N. This slow process, which is called beta decay, releases energy through the emission of electrons from the nucleus or positrons.

After approximately 5,730 years, half of the starting concentration of 14C will have been converted back to 14N. This is referred to as its half-life, or the time it takes for half of the original concentration of an isotope to decay back to its more stable form. Because the half-life of 14C is long, it is used to date formerly-living objects such as old bones or wood. Comparing the ratio of the 14C concentration found in an object to the amount of 14C in the atmosphere, the amount of the isotope that has not yet decayed can be determined. On the basis of this amount, the age of the material can be accurately calculated, as long as the material is believed to be less than 50,000 years old. This technique is called radiocarbon dating, or carbon dating for short.

Application of carbon dating: The age of carbon-containing remains less than 50,000 years old, such as this pygmy mammoth, can be determined using carbon dating.

Other elements have isotopes with different half lives. For example, 40K (potassium-40) has a half-life of 1.25 billion years, and 235U (uranium-235) has a half-life of about 700 million years. Scientists often use these other radioactive elements to date objects that are older than 50,000 years (the limit of carbon dating). Through the use of radiometric dating, scientists can study the age of fossils or other remains of extinct organisms.

Atomic Structure

Chlorine atom'>Atoms are the basic building blocks of everything around us. They come in different kinds, called elements, but each atom shares certain characteristics in common. All atoms have a dense central core called the atomic nucleus. Forming the nucleus are two kinds of particles: protons, which have a positive electrical charge, and neutrons, which have no charge. All atoms have at least one proton in their core, and the number of protons determines which kind of element an atom is. For example, an oxygen atom has 8 protons. If you were somehow able to change the proton number of this atom to 7, even if everything else remained the same, it would no longer be an oxygen atom, it would be nitrogen. For this reason, we list the different elements by their proton, or atomic, number. The periodic table of elements is a chart of all of the elements that have been discovered so far, in order by their atomic number.

In addition to protons and neutrons, all atoms have electrons, negatively charged particles that move around in the space surrounding the positively-charged nuclear core. Electrons are usually depicted in drawings as much smaller than protons or neutrons because their mass is so much smaller. In fact, electron mass is so small that it is not counted in an atom’s mass. However, the charge strength of a single electron is equal to that of a single proton, and despite their small mass, electrons are important for balancing the charge of an atom. Unless specifically stated otherwise, atoms always have the same number of electrons as protons; therefore, you can find the electron number by looking at the atomic number. But unlike protons, the number of electrons can and does change without affecting the kind of element an atom is!

We now know how to find the number of protons and the number of electrons for a given atom, but what about neutrons? How many neutrons do atoms of a given element have? It is NOT always the same as the number of protons and electrons. For example, hydrogen has one proton and one electron, but it doesn’t have any neutrons at all! We determine this by looking at the atomic mass. Even though an atom is so small it would take almost a million for you to see even a tiny dot on your computer screen, each tiny atom definitely has mass and occupies space. This mass comes from the nucleus. Each proton and neutron has about the same amount of mass, measured in daltons, or atomic mass units (amus). Because the unit of measure is defined by one proton, 1 proton = 1 neutron = 1 dalton = 1 amu. Electrons do have some mass, but it is almost 2000 times less than the mass of a proton. There aren’t enough electrons in any of the atoms we know about to affect the total mass; therefore, the total mass is equal to the sum of the protons and the neutrons in an atom.

Because we can find the number of protons and the atomic mass of an atom by looking at its element information in the periodic table, we can calculate the number of neutrons in that atom by subtracting the number of protons from the atomic mass.

When the number of neutrons is different for individual atoms of the same element, each atom is called an isotope. When you read a periodic table, the atomic mass listed is the average atomic mass for all of the isotopes of that element found in nature. For example, carbon has an atomic mass of 12.01 in the periodic table. Carbon can’t have 6.01 neutrons because you can’t have part of a neutron. The value exceeds 6 because, while most carbon atoms have 6 neutrons, some carbon atoms are found with 7 neutrons and others with 8 neutrons. For our purposes, we round the atomic mass to the nearest whole number to calculate the number of neutrons.

Atomic Structure

This video illustrates how atoms and their components work together.

All Atoms Are Made Up Of

Valence Electrons

Now that you’ve had a chance to work with atoms in general, let’s dig a little deeper. Electrons remain in an atom because of their attraction to the protons’ positive charge, but they are not as tightly associated with the atom as either protons or neutrons. Electrons are complicated particles because they have a lot of space to occupy in an atom, and yet they are also tied to a specific area within that atom. Although the drawings we have been working with show the nucleus as a medium-sized, visible object in the center of an atom, it is actually very tiny, and most of an atom is the space around the nucleus in which the electrons move.

Because of their shared negative charge, electrons repel one another if they get too close. At the same time, electrons are attracted to the positive charge of the nucleus. The details of the energy and position of electrons can get really complicated, but we will focus only on what we need to understand to study the molecules of life.

Sulfur atom'>Electrons are arranged in energy shells (also known as electron shells) around the atomic nucleus. Although electrons have plenty of space, they all want to be closest to the positive nuclear charge that is attracting them. At the same time, the electrons repel one another because of their negative charge, and only a few can get close to the nucleus at any given time. Practically speaking, only two electrons can fit in the three-dimensional space closest to the nucleus. This space is called the first energy shell. If there are three electrons in an atom, the first two will be found in the first energy shell. The third electron will have to settle for the second energy shell, a three-dimensional space a little farther from the nucleus, where it will be alone. In this example, the lone electron is called a valence electron, and the outermost energy shell that contains any electrons is called the valence shell.

Aluminum atom'>The second energy shell is big enough to hold as many as eight electrons, grouped in pairs inside four electron orbitals, or spaces where electrons spend most of their time. This means if there is only one electron in the second energy shell, there is a lot of extra space left.

When an energy shell is incompletely filled, the electron(s) in that shell are not as stable and are more likely to react. For this reason, atoms tend to react with other atoms in ways that will fill or empty their valence shell to gain the stability of a full outermost energy shell. Atoms can do this by gaining or losing electrons to become ions or by sharing electrons with other atoms to form stable associations.

Using electron number and energy shells, we can determine the number of valence electrons for any given atom and its expected level of reactivity. As you work with the example below, you should remember that although we draw energy shells as circles around an atomic nucleus, this is not meant to represent an actual electron path. The concentric circle style of drawing energy shells is meant to represent the average distance electrons in that energy shell are orbiting the nucleus. In reality, electrons do not move in a circular orbit as depicted in the drawing, but travel much more complicated pathways around an atomic nucleus.

Build an Atom

Use this activity to practice reading the periodic table to create several atoms.

What is the Valence?

All Atoms Are Quizlet

In this activity, you will calculate the number of valence electrons in atoms using the periodic table of elements.